| Positron annihilation technique (PAT) is a developed nuclear science and technology methods which are applied in the condensed matter physics, materials science and engineering, chemistry, biology science and engineering etc. It has incomparable advantages and prospects than the other methods. PAT contains positron annihilation lifetime spectroscopy (PALS), Doppler broadening spectroscopy (DBS), Positron angular correlation spectroscopy (ACAR), slow positron beam (SPB) etc. The advantages of this technique includes:First, it has a good sensitivity to the nano-scale defects in condensed matter. Second, it is a non-destructive methods. It doesn’t bring any new defects in the material. The methods to detect various parameters of positron annihilation such as:lifetime, momentum, energy and position are constantly improving, especially tends to the digital acquisition system which is flexibility in the use of the offline data processing. This technology progress makes a variety of data processing modes. In addition, various kinds of techniques to measure the related information in positron annihilation developed rapidly. This combination of techniques can overcome conventional single lost information making the micro-structure analysis incomplete. In polymer material science, the detection of the porous material structure properties with postronium(Ps) formation and annihilation mechanisms arouse more and more attention. How to accurately obtain the porous characteristic parameters has become the primary mission.The first task of this paper is to study the Ps formation and annihilation mechanisms in the porous material with a digital positron annihilation spectrometer and the following achievements are:1. We first successfully developed a new type of digital positron annihilation lifetime spectrometer based on the DRS4evaluation board and cerium-doped lanthanum bromide scintillators. In this study we respectively used the pulse baseline discriminator method, the pulse amplitude discriminator method, the pulse area discriminator method, the pulse rise time discriminator method to discriminate the pulse. Then we separately used the linear interpolation, Lagrange interpolation, Hermite interpolation, Gaussian fitting method, the cubic fitting method etc for the digital constant fraction discrimination. We tested a positron annihilation lifetime and discussed the means to improve the performance of the apparatus. This improvement makes the positron annihilation lifetime spectrometer more simple, small and practical.2. In order to improve the accuracy and intelligent of analysing the positron lifetime spectrum, we developed a new method-Back Propagation Network(BP) which is one of the Artifical Neural Networks to analysis the spectrum. This algorithm not only eliminates the need for presetting parameter in the conventional arithmetic but also can automatically identify the composition of the unknown lifetime spectrum, which makes the resolution of the spectrum more intelligent. It can give the exact value and distribution of the spectrum. In addition, we also studied the accuracy of analysing the spectrum with various parameters of the neural network structure. At last we generate a good neural network for solution in a wide range.The second task of this paper is to develop a two-dimensional positron annihilation lifetime spectrometer used the conventional liquid source and the following results were obtained:1. We used the commercial digital acquisition to build a new type of two-dimensional digital positron annihilation spectrometer. This instrument could record the energy and lifetime of the annihilation event simultaneously. By the two-dimensional mapping, we can research the energy information with the different lifetime and the lifetime information with the different energy. The time resolution of the spectrometer is226ps and the energy resolution is9.6%(at511keV). We compare the different detection methods make the different performance of the spectrometer and discuss the constraints and solution of the spectrometer.We use the spectrometer to study the Ps information in the different quality ratio of mixed nanopowders between γ-Al2O3and a-Fe2O3(0%,16wt%α-Fe2O3/γ-Al2O3,50wt%α-Fe2O3/y-Al2O3, a-Fe2O3). We want to explain the Ps mutual conversion issues from o-Ps to p-Ps in the porous material, and we also used the other conventional physical approach such as TEM, XRD, XRF, EPR to study the grain size and the unpaired electron concentration about the material. When we use the two-dimensional positron annihilation lifetime to measure the material, we found that with the increase of the unpaired electron concentration, the fourth lifetime and intensity is decreased sharply, but the third lifetime and intensity remain unchanged; we also found that the3y/2y ratio in the energy spectrum is decreased slowly with the increase of the unpaired electron concentration. The two information shows that o-Ps which create three photon annihilation is reduced, and proved that the unpaired... |
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